Extrusion is a well-known process that is used in many industries. A variety of products from foods to optical fibers, plastics and metals are produced by extrusion. In the extrusion process, the materials to be extruded are fed into the extruder, a type of giant mixer. The materials are mixed, heated if appropriate, and fed into a barrel that contains an internal rotating screw. The screw moves the liquid or viscous material being extruded (the “extrudate”), out the end of the extruder, through a die having the shape of the desired product. The extrudate will take the shape into which it is formed coming out through the extruder die. The extrudate is generally continuous, and is often cut into desired lengths after being extruded out the die. Extrudate is formed into an almost infinite variety of shapes, ranging from tubes to sheet material.
It has been found that the outside edges of the screw threads in the extruder, which mix the extrudate and push it out the end of the extrusion barrel, tend to be more subject to wear than other parts of the screw threads. When the outside edges of the screw threads wear sufficiently, extrudate flows between the screw threads and the barrel, which interferes with the normal extrusion process. To this end, steps have been taken to extend the durability on those parts of the screws that see additional wear. Such steps have included applying a coating of various liquid substances that harden on the screw or applying an additional layer of solid wear-resistant material to the portions of the extruder screw subject to wear. However, it has been found that materials that project out from the surface of the screw are subject to shear during use, and the materials can experience fatigue cracks. Subsequently, pieces of the material can break off and end up being incorporated in the extrudate being mixed. Often this necessitates the disposal of batches of extrudate which are contaminated with pieces of wear-resistant material or coating.
Another solution has been to cut grooves in the crests of the extruder screw threads, and insert other materials, such as molybdenum, in those grooves, as shown in U.S. Pat. No. 5,135,378 to Catton (“Catton”). The Catton process is, however, very complex, involving cutting a groove, laying in a first material along the sides and at intervals as bridges across the width of the groove, then laying in a second material in a liquid state that hardens to form a mechanical bond with the extruder screw. The added material is then ground to form a smooth surface.
Another solution, as disclosed in U.S. Pat. No. 6,346,293 to Douris et al. (“Douris”), is to weld a bead of a hardened material along part of the width of the crest of the extruder screw threads, and then weld beads of a different hardened material along either side of the first bead of hardened material. Subsequently, the hardened materials are ground to the required tolerances. Because of the many steps involved in welding two separate materials to the crest of the screw, because of the need for the materials that will bond to each other as well as to the screw thread, and because of the grinding the hardened material, the process is very involved and time consuming. Thus, the Douris process is rather expensive.
Therefore, a method and/or apparatus for extending the life of extruder screws is needed by making them more resistant to wear wherein the method does not compromise the structural integrity of the screw, nor produce fragments that contaminates the extrudate, and is not overly complex, involved or expensive.